Cognitive Radio (CR) is considered as a type of radio in which communications systems are aware of their environment as well as their internal state and may make decisions about their radio operating behavior based on that information and predefined objectives. CR users can utilize licensed spectrum bands when they do not cause interference to existing users (also referred to as licensed users (LUs) or primary users (PUs)). A time when CR users can transmit without causing interference to the primary users is called a spectrum opportunity. Clearly, the ability to sense the spectrum to detect spectrum opportunities is crucial to CR performance and has been the subject of considerable research.
The basic concept of CR is to allow unlicensed CR users, also called secondary users, to use licensed spectrum bands (also referred to as licensed frequency bands) as long as they do not cause interference to licensed users. Therefore, CR users must be able to identify and use spectrum bands that are not being used by primary users. In practice, the available spectrum bands for CR users may vary with time and location.